PROTEINS AS COLLOIDS 299 



to be charged as a whole either positively or negatively, and 

 accordingly when such a solution is subjected to a difference 

 of potential, the colloidal particles wander bodily to one or 

 other of the electrodes. 



Biltz has shown that two colloids mutually precipitate 

 each other only if they bear unlike charges, and when once 

 precipitated, they become electrically neutral and are no 

 longer transported by an electric current. It has further 

 been shown that only those crystalloids which are electrolytes 

 are able to precipitate colloids, such substances as urea or 

 cane sugar being unable to effect precipitation. 



According to Hardy and Bredig, the particles in a colloidal 

 solution are held in suspension by the opposing forces of capil- 

 lary attraction and of electrostatic repulsion such as must 

 exist between particles all of which bear the same charge ; 

 the precipitation of a colloid by the addition of an electro- 

 lyte is accordingly attributed to the elimination of the electro- 

 static repulsion by the fact of the charge borne by the ions of 

 the electrolyte neutralizing the charge borne by the particles. 

 Billitzer, on the other hand, holds the view that on introduc- 

 ing the electrolyte into the solution the particles tend to con- 

 gregate around the ions, and are thereby brought into such 

 close contact with each other that they form sufficiently large 

 aggregates to be precipitated. 



By means of conductivity experiments, Pauli * was able 

 to show that pure egg albumen, free from electrolytes by 

 repeated dialysis, was electrically neutral, for, on subjecting a 

 solution of this substance to an electric current for twenty-four 

 hours, no particles of albumen were transferred to either of the 

 two electrodes. He found, moreover, that the addition of 

 neutral salts of the alkali metals or the metals of the alkaline 

 earths, produced no change in the electrical state of the 

 albumen, whereas on adding traces of acids or acid salts the 

 albumen assumed a positive charge, while on the addition of 

 bases or salts having an alkaline reaction, it became electro- 

 negative. This observation that an electrolyte, such as hydro- 

 chloric acid, which contains an equal number of oppositely 

 charged ions, is able to impart a positive charge to electric- 



*Pauli: " Hofmeister's Beitrage," 1902, III, 225; 1904, V, 27; 1905, VI, 

 233, etc. 



